Mode-S Codes Set To Replace SSR Codes

A Lufthansa Boeing 737 recently became the first aircraft to use mode-S capability to transmit its radar identification (flight ID), without the use of a secondary surveillance radar (SSR) code. The 737 flight crew was able to automatically correlate the aircraft’s electronic flight plan data by entering a 24-bit code into the aircraft computer. Rod Marten, who oversees mode-S operations at Eurocontrol, said a 24-bit code is necessary because the short supply of four-digit SSR codes–4,096 in total–requires European controllers to make an average of 20,000 code changes a day and duplicate aircraft codes. There are approximately 17 million 24-bit codes, however.